Delay locked loop (DLL) circuits reside in many memory devices to receive an external signal to generate an internal signal. The DLL automatically keeps the internal signal synchronized with the external signal. The internal signal usually serves as a reference signal for the memory devices instead of the external signal because the internal signal matches internal operating conditions of the memory devices, such as process, voltage, and temperature, better than the external signal does.
A typical memory device has many memory cells to store data. The memory device writes data into the memory cells during a write mode and reads the data stored in the memory cells during a read mode. The memory device also has other modes of operations.
In some modes of operations, the memory device uses the internal clock signal generated by the DLL. In other modes of operations, the internal clock signal is unused. Therefore, keeping the DLL active all the time wastes power.